The present invention is directed to a rotor and, more particularly, is directed to a relatively large diameter rotor lid which in conjunction with the lower rotor portion automatically increases the sealing effect between the rotor lid and the lower portion of the rotor during centrifugation.
Certain types of rotors are designed for the centrifugation of very small samples which must be situated a relatively large distance with respect to the test tube size from the axis of rotation in order to achieve acceptable separation results during centrifugation. However, in order to properly isolate the fluid samples in the rotor from the vacuum environment which exists within the centrifuge, a lid must be sealed over the test tube cavities. Since the fluid samples are located a considerable distance from the spin axis of the rotor, it is necessary to utilize a relatively large diameter lid on the rotor.
Typically, rotor lids are generally smaller in diameter with respect to the outer diameter of the rotor. In addition, these rotor lids normally have a central aperture to receive a projecting shaft from the lower portion of the rotor. A handle or cap-like nut is screwed onto this projecting threaded shaft which can be tightened down sufficiently with a proper tightening tool to provide a seal between the outer perimeter of the lid and the outer perimeter of the lower portion of the rotor.
However, the larger the rotor lid having a center hole is, the greater the stress experienced on the lid during centrifugation. For example, the maximum material stress on a lid with a hole will be approximately twice that which would be experienced by the same sized lid having no hole. As the lid becomes larger in diameter, the effects of the hole with respect to the centrifugally induced forces during centrifugation becomes extremely great resulting in an undesirable design feature. Also, as the diameter of the lid increases it is difficult to tighten the center portion of the lid enough by the use of a threaded nut or handle over the stud from the lower end of the rotor. The necessary sealing contact between the outer perimeter of the rotor lid and the perimeter of the lower portion of the rotor is not established.
Another problem typically associated with the sealing between the lid and the rotor is proper retention of the O-ring in place between the lid and the rotor to maintain the seal. Quite often the design of the rotor and lid is not adequate to keep the O-ring in place during high speed rotation. In many instances the O-ring will creep away from its sealing position as a result of high centrifugally induced forces, adversely affecting the seal between the lid and the rotor.
The problems expressed above become more magnified as the speed to which the rotor is subjected is increased.